Epstein-Barr virus (EBV) is an important organism because it can cause a variety of alterations of the human host. It may induce an asymptomatic infection; lymphoproliferative disorder, infectious mononucleosis; a malignancy, lymphoma, probably with assistance of other as yet unknown factors; and a state of latency in the human host but which reactivates at intervals for reasons unknown. The main objective of this study is to prospectively investigate the transmission pattern and host response to EBV in families which contain an index case of primary EBV infection. As a secondary objective, the pattern of viral excretion and humoral immune response in children with manifest primary EBV infection will be documented. Interviews, blood, and oropharyngeal specimens will be obtained from the participating families at intervals over a period of 8 months after onset of the index case. The investigators expect to demonstrate important pathogenetic facts about the significance of viral reactivation in adults and the subsequent transmission of virus to susceptible family members. This study should increase the understanding of the host-virus relationship and will be of potential help in explaining why this ubiquitous virus may be associated with a variety of host responses including the initiation of a malignant process.